Do you want your images to have a fixed size?
If not:
<html>
<head>
<style>
img {
width: 33%;
height: 100px;
float: left;
}
</style>
</head>
<body>
<img src=""><img src=""><img src="">
<img src=""><img src=""><img src="">
<img src=""><img src=""><img src="">
</body>
</html>
If yes: (You might need to consider a fixed container width.)
<html>
<head>
<style>
.main-container {
width: 900px;
margin: 0 auto;
}
img {
width: 300px;
height: 200px;
float: left;
}
</style>
</head>
<body>
<div class="main-container">
<img src=""><img src=""><img src="">
<img src=""><img src=""><img src="">
<img src=""><img src=""><img src="">
</div>
</body>
</html>
Floating allows the images to fill a row. Fixed with of images and the container makes it possible to determine how many images there will be in the row.
If you want to follow responsive design, you could add break point to your page so that in smaller screen sizes your grid of images depends on available space.
Normally you will want to have fixed size for your images that is their actual size. Otherwise the displayed quality of the images might be bad.